Potent nonimmunosuppressive cyclophilin inhibitors with improved pharmaceutical properties and decreased transporter inhibition

Assessing Trans-Inhibition of OATP1B1 and OATP1B3 by Calcineurin and/or PPIase Inhibitors and Global Identification of OATP1B1/3-Associated Proteins

Organic anion transporting polypeptide (OATP) 1B1 and OATP1B3 are key determinants of drug-drug interactions (DDIs). Various drugs including the calcineurin inhibitor (CNI) cyclosporine A (CsA) exert preincubation-induced trans-inhibitory effects upon OATP1B1 and/or OATP1B3 (abbreviated as OATP1B1/3) by unknown mechanism(s). OATP1B1/3 are phosphoproteins; calcineurin, which dephosphorylates and regulates numerous phosphoproteins, has not previously been investigated in the context of preincubation-induced trans-inhibition of OATP1B1/3. Herein, we compare the trans-inhibitory effects exerted on OATP1B1 and OATP1B3 by CsA, the non-analogous CNI tacrolimus, and the non-CNI CsA analogue SCY-635 in transporter-overexpressing human embryonic kidney (HEK) 293 stable cell lines. Preincubation (10-60 min) with tacrolimus (1-10 µM) rapidly and significantly reduces OATP1B1- and OATP1B3-mediated transport up to 0.18 ± 0.03- and 0.20 ± 0.02-fold compared to the control, respectively. Both CsA and SCY-635 can trans-inhibit OATP1B1, with the inhibitory effects progressively increasing over a 60 min preincubation time. At each equivalent preincubation time, CsA has greater trans-inhibitory effects toward OATP1B1 than SCY-635. Preincubation with SCY-635 for 60 min yielded IC50 of 2.2 ± 1.4 µM against OATP1B1, which is ~18 fold greater than that of CsA (0.12 ± 0.04 µM). Furthermore, a proteomics-based screening for protein interactors was used to examine possible proteins and processes contributing to OATP1B1/3 regulation and preincubation-induced inhibition by CNIs and other drugs. A total of 861 and 357 proteins were identified as specifically associated with OATP1B1 and OATP1B3, respectively, including various protein kinases, ubiquitin-related enzymes, the tacrolimus (FK506)-binding proteins FKBP5 and FKBP8, and several known regulatory targets of calcineurin. The current study reports several novel findings that expand our understanding of impaired OATP1B1/3 function; these include preincubation-induced trans-inhibition of OATP1B1/3 by the CNI tacrolimus, greater preincubation-induced inhibition by CsA compared to its non-CNI analogue SCY-635, and association of OATP1B1/3 with various proteins relevant to established and candidate OATP1B1/3 regulatory processes.

Non-Immunosuppressive Cyclophilin Inhibitors

Cyclophilins, enzymes with peptidyl-prolyl cis/trans isomerase activity, are relevant to a large variety of biological processes. The most abundant member of this enzyme family, cyclophilin A, is the cellular receptor of the immunosuppressive drug cyclosporine A (CsA). As a consequence of the pathophysiological role of cyclophilins, particularly in viral infections, there is a broad interest in cyclophilin inhibition devoid of immunosuppressive activity. This Review first gives an introduction into the physiological and pathophysiological roles of cyclophilins. The presentation of non-immunosuppressive cyclophilin inhibitors will commence with drugs based on chemical modifications of CsA. The naturally occurring macrocyclic sanglifehrins have become other lead structures for cyclophilin-inhibiting drugs. Finally, de novo designed compounds, whose structures are not derived from or inspired by natural products, will be presented. Relevant synthetic concepts will be discussed, but the focus will also be on biochemical studies, structure-activity relationships, and clinical studies.

Repurposing of cyclophilin A inhibitors as broad-spectrum antiviral agents

Cyclophilin A (CypA) is linked to diverse human diseases including viral infections. With the worldwide emergence of severe acute respiratory coronavirus 2 (SARS-CoV-2), drug repurposing has been highlighted as a strategy with the potential to speed up antiviral development. Because CypA acts as a proviral component in hepatitis C virus, coronavirus and HIV, its inhibitors have been suggested as potential treatments for these infections. Here, we review the structure of cyclosporin A and sanglifehrin A analogs as well as synthetic micromolecules inhibiting CypA; and we discuss their broad-spectrum antiviral efficacy in the context of the virus lifecycle.

Cyclophilin A: a possible host modulator in Chandipura virus infection

Chandipura virus (CHPV), belonging to the genus Vesiculovirus of the family Rhabdoviridae, has been identified as one of the causes of pediatric encephalitis in India. Currently, neither vaccines nor therapeutic drugs are available against this agent. Considering that the disease progresses very fast with a high mortality rate, working towards the development of potential therapeutics against it will have a public health impact. Although the use of viral inhibitors as antiviral agents is the most common way to curb virus replication, the mutation-prone nature of viruses results in the development of resistance to antiviral agents. The recent development of proteomic platforms for analysis of purified viral agents has allowed certain upregulated host proteins that are involved in the morphogenesis and replication of viruses to be identified. Thus, the alternative approach of inhibition of host proteins involved in the regulation of virus replication could be explored for their therapeutic effectiveness. In the current study, we have evaluated the effect of inhibition of cyclophilin A (CypA), an immunophilin with peptidyl-prolyl cis/trans-isomerase activity, on the replication of CHPV. Treatment with cyclosporin A, used in vitro for the inhibition of CypA, resulted in a 3-log reduction in CHPV titer and an undetectable level of CypA in comparison to an untreated control. An in silico analysis of the interaction of the CHPV nucleoprotein with the human CypA protein showed stable interaction in molecular docking and molecular dynamics simulations. Overall, the results of this study suggest a possible role of CypA in facilitating CHPV replication, thus making it one of the potential host factors to be explored in future antiviral studies.

Cyclophilin A and CD147: novel therapeutic targets for the treatment of COVID-19

The outbreak of pneumonia caused by a new coronavirus (SARS-CoV-2) occurred in December 2019, and spread rapidly throughout the world. There have been other severe coronavirus outbreaks worldwide, namely, severe acute respiratory syndrome (SARS-CoV) and Middle East respiratory syndrome (MERS-CoV). Because the genetic diversity of coronaviruses renders the design of vaccines complicated, broad spectrum-anti-coronavirus drugs have become a critical approach to control the coronavirus epidemic. Cyclophilin A is an important protein needed for coronavirus replication, and its inhibitor cyclosporine A has the ability to suppress coronavirus on a broad spectrum. CD147-S protein was found to be one route by which SARS-CoV-2 invades host cells, while CD147 was found to play a functional role in facilitating the infection of host cells by SARS-CoV. The CyPA/CD147 interaction may play a critical role in the ability of the SARS-CoV-2 virus to enter the host cells. However, cyclosporine A has immunosuppressive effects, so the conditions for its use as an antiviral drug are limited. As a result, cyclosporine A analogues without immunosuppressive side effects have attracted lots of interest. This review primarily discusses the drug development prospects of cyclophilin A as a therapeutic target for the treatment of coronavirus infection, especially coronavirus disease 2019 (COVID-19), and non-immunosuppressive cyclosporine analogues.

Targeting Protein Folding: A Novel Approach for the Treatment of Pathogenic Bacteria

Infectious diseases are a major cause of morbidity and mortality worldwide, exacerbated by increasing antibiotic resistance in many bacterial species. The development of drugs with new modes of action is essential. A leading strategy is antivirulence, with the aim to target bacterial proteins that are important in disease causation and progression but do not affect growth, resulting in reduced selective pressure for resistance. Immunophilins, a superfamily of peptidyl-prolyl cis-trans isomerase (PPIase) enzymes have been shown to be important for virulence in a broad-spectrum of pathogenic bacteria. This Perspective will provide an overview of the recent advances made in understanding the role of each immunophilin family, cyclophilins, FK506 binding proteins (FKBPs), and parvulins in bacteria. Inhibitor design and medicinal chemistry strategies for development of novel drugs against bacterial FKBPs will be discussed. Furthermore, drugs against human cyclophilins and parvulins will be reviewed in their current indication as antiviral and anticancer therapies.

Discovery of a novel 9-position modified second-generation anti-HCV candidate via bioconversion and semi-synthesis of FR901459

Evidence that hepatitis C virus (HCV) utilizes cellular cyclophilin proteins in the virus replication cycle has increased attention on cyclophilin inhibitors as attractive therapeutic targets in the treatment of HCV. Previous reports have described a number of non-immunosuppressive cyclophilin inhibitors, most of which require many synthetic steps for their preparation. Sasamura et al. have previously reported the isolation of bioconversion derivative 4. This analog is a convenient starting point for optimization due to the presence of the readily modifiable primary hydroxyl group and because it shows moderate anti-HCV activity and decreased immunosuppressive activity. We have also established an efficient C-alkylation reaction at the 3-position. Through a detailed structure-activity relationship study, we discovered a new type of clinical candidate 14 which requires a short synthetic process and has potent anti-HCV activity and reduced immunosuppressive activity, as well as improved aqueous solubility and pharmacokinetics.

Development of an efficient semisynthetic modification of FR901459 via a novel regioselective N, O-acyl migration

HCV utilizes cellular protein cyclophilins in the virus replication cycle and cyclophilin inhibitors have become a new class of anti-HCV agents. In our screening of natural products, we identified a unique cyclosporin analogue, FR901459, as a cyclophilin inhibitor with potent anti-HCV activity. In this work, we developed an efficient synthetic methodology to prepare FR901459 derivatives via an N, O-acyl migration reaction. This method allows us to efficiently manipulate the amino acid residues at the 3 position while avoiding lengthy total synthesis for each compound. By using this methodology, we discovered 4, which has superior anti-HCV activity and decreased immunosuppressive activity compared to FR901459.

Systemic delivery of peptides by the oral route: Formulation and medicinal chemistry approaches

In its 33 years, ADDR has published regularly on the po5tential of oral delivery of biologics especially peptides and proteins. In the intervening period, analysis of the preclinical and clinical trial failures of many purported platform technologies has led to reflection on the true status of the field and reigning in of expectations. Oral formulations of semaglutide, octreotide, and salmon calcitonin have completed Phase III trials, with oral semaglutide being approved by the FDA in 2019. The progress made with oral peptide formulations based on traditional permeation enhancers is against a background of low and variable oral bioavailability values of ~1%, leading to a current perception that only potent peptides with a viable cost of synthesis can be realistically considered. Desirable features of candidates should include a large therapeutic index, some stability in the GI tract, a long elimination half-life, and a relatively low clearance rate. Administration in nanoparticle formats have largely disappointed, with few prototypes reaching clinical trials: insufficient particle loading, lack of controlled release, low epithelial particle uptake, and lack of scalable synthesis being the main reasons for discontinuation. Disruptive technologies based on engineered devices promise improvements, but scale-up and toxicology aspects are issues to address. In parallel, medicinal chemists are synthesizing stable hydrophobic macrocyclic candidate peptides of lower molecular weight and with potential for greater oral bioavailability than linear peptides, but perhaps without the same requirement for elaborate drug delivery systems. In summary, while there have been advances in understanding the limitations of peptides for oral delivery, low membrane permeability, metabolism, and high clearance rates continue to hamper progress.

A Pan-Cyclophilin Inhibitor, CRV431, Decreases Fibrosis and Tumor Development in Chronic Liver Disease Models

Previous studies show that cyclophilins contribute to many pathologic processes, and cyclophilin inhibitors demonstrate therapeutic activities in many experimental models. However, no drug with cyclophilin inhibition as the primary mode of action has advanced completely through clinical development to market. In this study, we present findings on the cyclophilin inhibitor, CRV431, that highlight its potential as a drug candidate for chronic liver diseases. CRV431 was found to potently inhibit all cyclophilin isoforms tested—A, B, D, and G. Inhibitory constant or IC₅₀ values ranged from 1 to 7 nM, which was up to 13 times more potent than the parent compound, cyclosporine A (CsA), from which CRV431 was derived. Other CRV431 advantages over CsA as a nontransplant drug candidate were significantly diminished immunosuppressive activity, less drug transporter inhibition, and reduced cytotoxicity potential. Oral dosing to mice and rats led to good blood exposures and a 5- to 15-fold accumulation of CRV431 in liver compared with blood concentrations across a wide range of CRV431 dosing levels. Most importantly, CRV431 decreased liver fibrosis in a 6-week carbon tetrachloride model and in a mouse model of nonalcoholic steatohepatitis (NASH). Additionally, CRV431 administration during a late, oncogenic stage of the NASH disease model resulted in a 50% reduction in the number and size of liver tumors. These findings are consistent with CRV431 targeting fibrosis and cancer through multiple, cyclophilin-mediated mechanisms and support the development of CRV431 as a safe and effective drug candidate for liver diseases.

Site Selective Amide Reduction of Cyclosporine A Enables Diverse Derivation of an Important Cyclic Peptide

Site selective amide reductions of the cyclic undecapeptide, cyclosporine A, have been developed using the combination of a heteroleptic borane catalyst and a silane reductant. Tertiary silane Me₂EtSiH provides two unique cyclosporine A derivatives, one of which can be readily diversified in subsequent reactions. The secondary silane Et₂SiH₂ enables divergent reactivity that uses a free hydroxyl group to direct the reduction. The transient O-silyl hemiaminal intermediate of this reduction can additionally be trapped by reducing to the amine or by reductive cyanation.

X-ray structure of alisporivir in complex with cyclophilin A at 1.5 Å resolution

Alisporivir (ALV) is an 11-amino-acid hydrophobic cyclic peptide with N-methyl-D-alanine and N-ethyl-L-valine (NEV) residues at positions 3 and 4, respectively. ALV is a non-immunosuppressive cyclosporin A (CsA) derivative. This inhibitor targets cyclophilins (Cyps), a family of proteins with peptidyl-prolyl cis/trans isomerase enzymatic activity. Cyps act as protein chaperones and are involved in numerous cellular functions. Moreover, Cyps have been shown to be an essential cofactor for the replication of many viruses, including Hepatitis C virus and Human immunodeficiency virus, and have also been shown to be involved in mitochondrial diseases. For these reasons, cyclophilins represent an attractive drug target. The structure of ALV in complex with cyclophilin A (CypA), the most abundant Cyp in humans, has been determined at 1.5 Å resolution. This first structure of the CypA-ALV complex shows that the binding of ALV is highly similar to that of CsA. The high resolution allowed the unambiguous determination of the conformations of residues 3 and 4 in ALV when bound to its target. In particular, the side-chain conformation of NEV4 precludes the interaction of the CypA-ALV complex with calcineurin, a cellular protein phosphatase involved in the immune response, which explains the non-immunosuppressive property of ALV. This study provides detailed molecular insights into the CypA-ALV interaction.

Discovery of a Potent and Orally Bioavailable Cyclophilin Inhibitor Derived from the Sanglifehrin Macrocycle

Cyclophilins are a family of peptidyl-prolyl isomerases that are implicated in a wide range of diseases including hepatitis C. Our aim was to discover through total synthesis an orally bioavailable, non-immunosuppressive cyclophilin (Cyp) inhibitor with potent anti-hepatitis C virus (HCV) activity that could serve as part of an all oral antiviral combination therapy. An initial lead 2 derived from the sanglifehrin A macrocycle was optimized using structure based design to produce a potent and orally bioavailable inhibitor 3. The macrocycle ring size was reduced by one atom, and an internal hydrogen bond drove improved permeability and drug-like properties. 3 demonstrates potent Cyp inhibition ( K_d = 5 nM), potent anti-HCV 2a activity (EC₅₀ = 98 nM), and high oral bioavailability in rat (100%) and dog (55%). The synthetic accessibility and properties of 3 support its potential as an anti-HCV agent and for interrogating the role of Cyp inhibition in a variety of diseases.

Quercetin‑3‑O‑β‑D‑glucoside decreases the bioavailability of cyclosporin A through regulation of drug metabolizing enzymes, transporters and nuclear receptors in rats

Quercetin is a flavonoid compound that is widely present in food and drink. Quercetin-3-O-β-D-glucoside (Q3GA) is a major metabolite of quercetin. The aim of the present study was to investigate the effect of Q3GA on the pharmacokinetics of orally and intravenously administered cyclosporin A (CsA) in rats, and to assess the effect of Q3GA on drug-metabolizing enzymes (DMEs), drug transporters (DTs) and nuclear receptors (NRs). The pharmacokinetic parameters of CsA were measured following oral (10 mg/kg) and intravenous (2.5 mg/kg) administration of CsA in the presence or absence of Q3GA. The mRNA and protein expression levels of DMEs, DTs and NRs in the liver and small intestine were detected by quantitative polymerase chain reaction and western blot analysis. The results indicated that the intravenous administration of Q3GA (2.5, 5 or 10 mg/kg) for 7 consecutive days reduced the bioavailability of oral CsA. By contrast, the pharmacokinetics of the intravenous administration of CsA were not affected by Q3GA. However, the mRNA and protein expression levels of DMEs and DTs were inhibited by Q3GA. The activation of DMEs and DTs by NRs, and the interplay between DMEs and DTs, may explain these results. The present study identified a novel flavonoid-drug interaction, which may have implications for patients taking CsA and quercetin supplements or on a quercetin-containing diet.

Current therapy for chronic hepatitis C: The role of direct-acting antivirals

One of the most exciting developments in antiviral research has been the discovery of the direct-acting antivirals (DAAs) that effectively cure chronic hepatitis C virus (HCV) infections. Based on more than 100 clinical trials and real-world studies, we provide a comprehensive overview of FDA-approved therapies and newly discovered anti-HCV agents with a special focus on drug efficacy, mechanisms of action, and safety. We show that HCV drug development has advanced in multiple aspects: (i) interferon-based regimens were replaced by interferon-free regimens; (ii) genotype-specific drugs evolved to drugs for all HCV genotypes; (iii) therapies based upon multiple pills per day were simplified to a single pill per day; (iv) drug potency increased from moderate (∼60%) to high (>90%) levels of sustained virologic responses; (v) treatment durations were shortened from 48 to 12 or 8 weeks; and (vi) therapies could be administered orally regardless of prior treatment history and cirrhotic status. However, despite these remarkable achievements made in HCV drug discovery, challenges remain in the management of difficult-to-treat patients.

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HCV genotype-specific drugs evolve to pan-genotypic drugs.

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Drug potency increases from moderate (∼60%) to high (>90%) levels of sustained virologic response.

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Treatment durations are shortened from a 48-week to 12-week or 8-week period.

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HCV therapies based upon multiple pills per day are simplified to a single pill per day.

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HCV therapies are administered orally regardless of prior treatment history and cirrhotic status.

The mitochondrial permeability transition pore in AD 2016: An update

Over the past 30years the mitochondrial permeability transition - the permeabilization of the inner mitochondrial membrane due to the opening of a wide pore - has progressed from being considered a curious artifact induced in isolated mitochondria by Ca(2+) and phosphate to a key cell-death-inducing process in several major pathologies. Its relevance is by now universally acknowledged and a pharmacology targeting the phenomenon is being developed. The molecular nature of the pore remains to this day uncertain, but progress has recently been made with the identification of the FOF1 ATP synthase as the probable proteic substrate. Researchers sharing this conviction are however divided into two camps: these believing that only the ATP synthase dimers or oligomers can form the pore, presumably in the contact region between monomers, and those who consider that the ring-forming c subunits in the FO sector actually constitute the walls of the pore. The latest development is the emergence of a new candidate: Spastic Paraplegia 7 (SPG7), a mitochondrial AAA-type membrane protease which forms a 6-stave barrel. This review summarizes recent developments of research on the pathophysiological relevance and on the molecular nature of the mitochondrial permeability transition pore. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Impact of quercetin‑induced changes in drug‑metabolizing enzyme and transporter expression on the pharmacokinetics of cyclosporine in rats

The aim of the present study was to evaluate whether quercetin (Que) modulates the mRNA and protein expression levels of drug-metabolizing enzymes (DMEs) and drug transporters (DTs) in the small intestine and liver, and thus modifies the pharmacokinetic profile of cyclosporine (CsA) in rats. This two-part study evaluated the pharmacokinetic profiles of CsA in the presence or absence of Que (experiment I) and the involvement of DMEs and DTs (experiment II). In experiment I, 24 rats received single-dose CsA (10 mg/kg) on day 1, single-dose Que (25, 50 and 100 mg/kg/day; eight rats in each group) on days 3–8, and concomitant CsA/Que on day 9. In experiment II, the mRNA and protein expression levels of cytochrome P (CYP)3A1, CYP3A2, UDP glucuronosyltransferase family 1 member A complex locus, organic anion-transporting polypeptide (OATP)2B1, OATP1B2, P-glycoprotein, breast cancer resistance protein, and multidrug resistance-associated protein 2 in the small intestine and liver of rats were analyzed following oral administration of Que at 25, 50 and 100 mg/kg in the presence or absence of CsA (10 mg/kg) for seven consecutive days. Co-administration of Que (25,50 and 100 mg/kg) decreased the maximum serum concentration of CsA by 46, 50 and 47% in a dose-independent manner. In addition, the area under the curve to the last measurable concentration and area under the curve to infinite time were decreased, by 21 and 16%, 30 and 33%, and 33 and 34% (P<0.01), respectively. However, the mRNA and protein expression levels of the above-mentioned DMEs and DTs were inhibited by Que in a dose-dependent manner (P<0.01) to a similar extent in the small intestine and liver. It was demonstrated that Que was able to reduce the bioavailability of CsA following multiple concomitant doses in rats. Overlapping modulation of intestinal and hepatic DMEs and DTs, as well as the DME-DT interplay are potential explanations for these observations.

Antagonism of human formyl peptide receptor 1 with natural compounds and their synthetic derivatives

Formyl peptide receptor 1 (FPR1) regulates a wide variety of neutrophil functional responses and plays an important role in inflammation and the pathogenesis of various diseases. To date, a variety of natural and synthetic molecules have been identified as FPR1 ligands. Here, we review current knowledge on natural products and natural product-inspired small molecules reported to antagonize and/or inhibit the FPR1-mediated responses. Based on this literature, additional screening of selected commercially available natural compounds for their ability to inhibit fMLF-induced Ca(2+) mobilization in human neutrophils and FPR1 transfected HL-60 cells, and pharmacophore modeling, natural products with potential as FPR1 antagonists are considered and discussed in this review. The identification and characterization of natural products that antagonize FPR1 activity may have potential for the development of novel therapeutics to limit or alter the outcome of inflammatory processes.

Novel perspectives for hepatitis A virus therapy revealed by comparative analysis of hepatitis C virus and hepatitis A virus RNA replication

Hepatitis A virus (HAV) and hepatitis C virus (HCV) are two positive-strand RNA viruses sharing a similar biology, but causing opposing infection outcomes, with HAV always being cleared and HCV establishing persistence in the majority of infections. To gain deeper insight into determinants of replication, persistence, and treatment, we established a homogenous cell-culture model allowing a thorough comparison of RNA replication of both viruses. By screening different human liver-derived cell lines with subgenomic reporter replicons of HAV as well as of different HCV genotypes, we found that Huh7-Lunet cells supported HAV- and HCV-RNA replication with similar efficiency and limited interference between both replicases. HAV and HCV replicons were similarly sensitive to interferon (IFN), but differed in their ability to establish persistent replication in cell culture. In contrast to HCV, HAV replicated independently from microRNA-122 and phosphatidylinositol 4-kinase IIIα and β (PI4KIII). Both viruses were efficiently inhibited by cyclosporin A and NIM811, a nonimmunosuppressive analog thereof, suggesting an overlapping dependency on cyclophilins for replication. However, analysis of a broader set of inhibitors revealed that, in contrast to HCV, HAV does not depend on cyclophilin A, but rather on adenosine-triphosphate-binding cassette transporters and FK506-binding proteins. Finally, silibinin, but not its modified intravenous formulation, efficiently inhibited HAV genome replication in vitro, suggesting oral silibinin as a potential therapeutic option for HAV infections.

CONCLUSION

We established a cell-culture model enabling comparative studies on RNA replication of HAV and HCV in a homogenous cellular background with comparable replication efficiency. We thereby identified new host cell targets and potential treatment options for HAV and set the ground for future studies to unravel determinants of clearance and persistence.

Semi-synthesis of cyclosporins

BACKGROUND

Since its isolation in 1970, and discovery of its potent inhibitory activity on T-cell proliferation, cyclosporin A (CsA) has been shown to play a significant role in diverse fields of biology. Furthermore, chemical modification of CsA has led to analogs with distinct biological activities associated with its protein receptor family, cyclophilins.

SCOPE OF REVIEW

This review systematically collates the synthetic chemistry performed at each of the eleven amino acids, and provides examples of the utility of such transformations. The various modifications of CsA are traced from early, modest chemistry performed at the unique Bmt residue, through the remarkable use of a polyanion enolate that can be stereoselectively manipulated, and onto application of more recently developed olefin metathesis chemistry to prepare new CsA derivatives with unexpected biological activity.

MAJOR CONCLUSIONS

The myriad biological activities of CsA and its synthetic derivatives have inspired the development of new approaches to modify the CsA ring. In turn, these new CsA derivatives have served as tools in the discovery of new roles for cyclophilins.

GENERAL SIGNIFICANCE

This review provides information on the types of cyclosporin derivatives that are available to the many biologists working in this field, and should be of value to the medicinal chemist trying to discover drugs based on CsA. This article is part of a Special Issue entitled Proline-directed foldases: Cell signaling catalysts and drug targets.